1. Field of the Invention
The present invention relates to a motor control device that performs vector control to control a q-axis current and a d-axis current of a permanent magnet synchronous motor independent from each other in order to generate a torque of a permanent magnet synchronous motor having a stator provided with one of a group of windings and a group of permanent magnets and a rotor provided with the other thereof in a quick and stable manner.
2. Description of Related Art
A motor control device that performs vector control controls, for example, a motor in which an AC voltage from a power source (for example, three-phase AC power source) is applied through a converter, a smoothing capacitor and an inverter. In this case, the motor control device detects a q-axis current and a d-axis current of a permanent magnet synchronous motor based on currents (for example, U-phase current, V-phase current, and W-phase current) flowing through the motor and a position (for example, rotation angle) of a rotor of the motor, and generates a q-axis current command value and a d-axis current command value based on various kinds of commands from an upper control circuit (for example, CNC (computer numerical control)), etc. Then, the motor control device generates a PWM signal based on a difference between the detected q-axis current and the q-axis current command value and a difference between the detected d-axis current and the d-axis current command value and outputs the PWM signal to an inverter to drive the motor.
Conventionally, in a motor control device that performs vector control, in order to make it possible to drive a motor with high efficiency even at high speed when the iron loss becomes large, a motor control device which generates a d-axis current command value, in which loss of iron and copper of a motor is minimum, is proposed, as described in, for example, Japanese Unexamined Patent Publication (Kokai) No. 2008-236948 (JP2008-236948A).
When a permanent magnet synchronous motor in which permanent magnets are provided in one of a stator and a rotor is controlled by a motor control device, thermal demagnetization of the permanent magnets occurs due to overheating, and thus there may be a case where it is no longer possible for the permanent magnet synchronous motor to generate a torque in a quick and stable manner. Consequently, in order to avoid the state where thermal demagnetization of the permanent magnets occurs due to overheating, it is necessary to keep as small as possible an amount of rise in the temperature of the permanent magnet in a steady state of the permanent magnet synchronous motor.
However, in a conventional motor control device that generates a d-axis current command value, in which the loss of iron and copper is minimum, there may be a case where the permanent magnets enter an overheating state and thermal demagnetization of the permanent magnets occurs when the loss concentrates on the stator or rotor provided with the permanents magnet.